1. Field of the Invention
The present invention relates to keyboard switch assemblies which provide for momentary electrical contact in response to actuation of keys, and more particularly to keyboard switch assemblies of the type employing printed circuit patterns on thin, planar substrate materials which are momentarily driven into contact by depression of spring loaded pushbuttons.
2. History of the Prior Art
In order to be competitive in a high volume market such as the calculator market or the telephone market a keyboard switch assembly must satisfy a large number of conflicting demands. It should be compact, lightweight, very inexpensive, and highly reliable and have a satisfying touch to the operator. While electronic debouncing circuits are available, the economics of a particular use or the requirements of a particular customer frequently necessitate the use of a virtually bounce-free switch assembly. That is, a single, unambiguous contact closure signal is required for each activation of a keyboard key. The problem of providing a bounce-free operation becomes worse in multi-pole switch assemblies such as arrangements wherein a single key actuation must connect separate row and column conductors to a common voltage. Bounce-free operation is one factor amoung several which combine to define a force-displacement characteristic for the key. The characteristic relates to other factors as well, such as ease of depression of the key, tactile feel and percentage contact. Ease of depression of the key is generally desirable so as to bring this parameter within a desirable force range. For example, telephone companies often specify that the keys of a tone generating telephone should be operated by a force of 100-200 grams. In addition to reducing the bounce to a minimum so as to avoid generation of more than the single signal desired with each key depression, it is desirable to reduce the tactile feel of the key. Percentage of travel before contact is dictated by such things as application of the keyboard. For example, in the case of tone generating telephones, a desirable percentage of travel before contact might be such that the switch actuates after key displacement is approximately 50% of maximum travel and releases when the key displacement becomes less than about 30% of its maximum travel.
Some of the various factors involved in the successful operation of keyboard contacts are discussed in detail in an article by R. L. Deninger entitled "Human Factors Engineering Studies of the Design and Use of Pushbutton Telephone Sets", Bell System Technical Journal, July 1960, p. 995 et seq. Other factors such as contact arcing are discussed in an article entitled "Making and Breaking of Circuits is Analyzed by a Dozen Experts" in Product Engineering, December 1975, p. 37 et seq.
A variety of keyboard switch assembly arrangements have been developed in an attempt to meet the demands of the keyboard market. These include arrangements described in the following U.S. Pat. Nos. 3,699,294 to Sudduth, 3,783,205 to Boulanger, 3,780,237 to Seeger, Jr. et al, and 3,860,771 to Lynn et al. The Sudduth patent discusses the problem of bounce and minimization thereof. In the Seeger, Jr. et al patent each key is provided with a slideable plunger and included spring in addition to the main key spring to achieve certain advantages including minimization of damage to the moving parts of the switch resulting from excessive force or crushing. The arrangements shown in the patents to Lynn et al and Boulanger are typical of the state of the art in keyboard switch assemblies of the type used in tone producing telephones and in calculators. Such arrangements employ a laminate of different materials including printed circuit contacts thereon in conjunction with an actuator assembly including keys arranged so as to provide contact between selected ones of the printed circuit contacts when the keys are depressed. In such arrangements the laminate includes a separate bubble or dome-shaped protrusion in a flexible member disposed under each key such that the protrusion is eventually inverted with increasing pressure on the key to facilitate contact of selected ones of the printed circuit contacts.
While keyboard switch assemblies such as those shown in Lynn et al and Boulanger patents function reasonably well in most respects and for many applications, more extensive use and increased requirements for such equipment have rendered such arrangements generally unsuitable in many instances. Such arrangements, for example, have keys which are too difficult to depress for some applications, have undesirable force-displacement characteristics, have excessive bounce for certain applications and excessive tactile feel for certain applications. Such problems are due in part to the fact that the protrusions must be designed for and must carry a printed circuit pattern at the underside thereof forming sets of contacts for mating engagement with the sets of switch contacts on the substrate. These problems are further added to by the need to make the protrusion forming layer of non-conductive material of a type to which the printed circuit pattern is easily applied.
Accordingly, it would be advantageous to provide an improved keyboard switch assembly.
More specifically, it would be advantageous to provide a keyboard switch assembly having bubble protrusions designed and formed of any appropriate material to optimize the depression and release characteristics thereof without the need to carry printed circuit contacts thereon.